Compensation for unmeasurable inspiratory flow in a critical care ventilator

ABSTRACT

A system and method for operating a ventilator to compensate for external gas flow reaching a patient from an external device, such as a nebulizer. A control unit of the ventilator monitors the gas flow rate from the ventilator and compares the gas flow rate from the ventilator to an expired gas flow rate from the patient. The difference between the inspired flow rate and the expired flow rate is due to the external device. The control unit modifies the operation of the ventilator to compensate for the external gas flow rate such that the flow of gas reaching the patient is maintained at a desired level.

BACKGROUND OF THE INVENTION

The present disclosure generally relates to a method and system forproviding ventilator therapy to a patient. More specifically, thepresent disclosure relates to a method of adjusting the operation of aventilator to compensate for the flow of gas created by an external gassource, such as a nebulizer, to improve the ventilation therapy.

Clinicians commonly utilize a nebulizer to provide aerosoled drugdelivery to a patient that is connected to a ventilator. Nebulizers aretypically placed in the inspiratory limb of a patient circuit and areused to inject an aerosoled drug directly into the flow stream of thebreathing gases for the patient. Nebulizers are typically pneumatic orultrasonic technology-based devices that are run continuously for aperiod of time until delivery of discrete doses of the drug or agenthave been completed.

When utilizing a pneumatic nebulizer, the nebulized agent supplied tothe patient is received from the nebulizer entrained in a nebulizer gasflow. The nebulizer gas flow including the entrained nebulized agent isadded to the flow of gas from the ventilator such that the combined gasflow is provided to the patient during the inspiratory phase.

Although the use of a nebulizer to introduce a nebulized agent into theflow of gas to the patient functions well to deliver the nebulizedagent, the addition of the nebulizer gas flow to the gas flow from theventilator can create problems. In most modern ventilators, the flow ofgas from the nebulizer is added downstream from the sensors that measurethe inspiratory gas flow from the ventilator. In such an embodiment, thegas flow measured during the expiratory phase is higher than thecorresponding inspiratory volume from the ventilator. The increasedvolume of the expiratory gas flow can lead to unnecessary alarmconditions in the ventilator. Without some type of compensation for theflow provided by the nebulizer or other external device, the ventilatorcan generate unnecessary alarms, which significantly reduces thereliability of the volumes measured during the inspiratory andexpiratory phase. Alternatively, the patient could be over-ventilateddue to the unmeasured inspiratory flow from the nebulizer.

SUMMARY OF THE INVENTION

The present disclosure relates to a method and system for operating aventilator to compensate for external gas flows being provided to thepatient downstream from the ventilator. The method and system allows theventilator to compensate for the external gas flow rates without havingto receive any information directly from the external device deliveringthe additional gas flow.

The ventilator includes a control unit that operates one or more valvesto control the flow rate of one or more gases from the ventilator. Aninspiratory flow sensor is included in the ventilator that allows thecontrol unit to monitor the flow rate of gas leaving the ventilator. Theventilator further includes an expiratory flow sensor that receives andmeasures the flow of gas in the expiratory limb, which includes the flowfrom the patient during exhalation and any portion of the flow from theinspiratory limb that bypasses the patient. During normal operationwithout any external gas flow being provided to the patient other thanfrom the ventilator, the net volume of gas measured over a breath cycleis approximately zero.

When an external device, such as a nebulizer, provides an external gasflow to the patient in addition to the gas flow from the ventilator, thedifference between the flow rate determined by the inspiratory flowsensor and the flow rate determined by the expiratory flow sensor willnot be zero due to the unmeasured external gas flow. The value of thenet flow rate represents the gas flow from the external device, such asthe nebulizer. Since the gas flow from the external device is injectedinto the gas flow downstream from the ventilator, the actual volume ofgas inspired by the patient during each breath cycle is above a desiredtidal volume set by the operator.

The control unit operates one or more flow valves contained within theventilator to reduce the flow rate from the ventilator by the determinedflow rate from the nebulizer. In this manner, the combined flow from theventilator and the nebulizer will equal the desired inspiratory flowrate. By determining the external flow rate, the ventilator can reducethe flow from the ventilator and thus compensate for the external flowrate such that the actual flow rate of gas delivered to the patient ismaintained at a desired level.

Various other features, objects and advantages of the invention will bemade apparent from the following description taken together with thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate the best mode presently contemplated of carryingout the disclosure. In the drawings:

FIG. 1 is a schematic illustration of a mechanical ventilator andnebulizer for ventilating a patient; and

FIG. 2 is a flow chart illustrating the steps for adjusting the flowrate from the ventilator to compensate for the flow rate from anebulizer.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a typical configuration of a ventilation system forproviding a flow of ventilation gas to a patient 10. A ventilator 12provides a flow of ventilation gas through an inspiratory limb 14 to thepatient 10 through a patient conduit 16. Exhaled gases from the patienttravel through the patient conduit 16 and through an expiratory limb 18back to the ventilator 12. The expired gases from the patient are ventedto atmosphere as indicated by the ventilation discharge 20. Theinspiratory limb 14 and the expiratory limb 18 are connected to thepatient conduit 16 through a wye piece 22. The patient can receive theflow of ventilation gas through various different interfaces such as agas mask or a tracheal tube.

In the embodiment shown in FIG. 1, a nebulizer 24 is provided tomedicate the patient on the ventilator. The nebulizer 24 introducesaerosolized medical agent into the flow of gas from the ventilator 12 inthe inspiratory limb 14. The nebulized medical agent is entrained in aflow of gas in the nebulizer conduit 26. The nebulizer gas flow andnebulized agent combine with the flow of gas from the ventilator in theinspiratory limb 14 such that the combined gas flow is provided to thepatient through the patient conduit 16.

Although a nebulizer 24 is shown in the embodiment of FIG. 1, it shouldbe understood that other types of external devices could be utilized tointroduce an external gas flow into the inspiratory limb 14 forventilation therapy purposes. As an example, an external device thatinjects a vasal dilator could be utilized in place of the nebulizer 24.Such an external device provides an external flow of gas that is inaddition to the gas flow from the ventilator 12.

The ventilator 12 shown in the embodiment of FIG. 1 is only one type ofventilator that could be utilized while operating within the scope ofthe present disclosure. Although the specific operating components ofthe ventilator 12 will be shown and described, it should be understoodthat various different types of ventilators, including differentoperating components, could be utilized while operating within the scopeof the present disclosure.

In the embodiment shown in FIG. 1, the ventilator 12 receives air inconduit 28 from an appropriate source, not shown, such as a cylinder ofpressurized air or a hospital air supply manifold. Ventilator 12 alsoreceives pressurized oxygen in conduit 30, also from an appropriatesource, not shown, such as a cylinder or manifold. The flow of air inventilator 12 is measured by an air flow sensor 32 and controlled by anair flow valve 34. The flow of oxygen is measured by an oxygen sensor 36and controlled by an oxygen valve 38. The operation of the valves 34, 38is established by a control unit 40, such as a central processing unit(CPU) in the ventilator 12.

The air and oxygen are mixed in conduit 42 and pass through aninspiratory flow sensor 44 before exiting the ventilator 12 into theinspiratory limb 14. In the ventilator 12 shown in FIG. 1, theventilator 12 includes both an air flow sensor 32 and an oxygen flowsensor 36 in addition to the inspiratory flow sensor 44. It should beunderstood that the air flow sensor 32 and the oxygen flow sensor 36could be eliminated. The inspiratory flow sensor 44 provides anindication of the flow of the combined gas being delivered by theventilator 12 to the inspiratory limb 14.

The expiratory limb 18 from the patient is also received at theventilator 12 and the expired gas from the patient and any bypass flowfrom the inspiratory limb 14 passes through an expiratory flow sensor 46before being discharged from the ventilator 12. The expiratory flowsensor 46 is in communication with the control unit 40 such that thecontrol unit 40 can monitor the expiratory gas flow from the patient.

Ventilator 12 further includes a display 48 to provide visual indicatorsto medical personnel as to the current operating parameters for theventilator 12. A user input device 50 allows the user to input operatingparameters to the ventilator, as desired. One example of a ventilator 12operating in accordance with the embodiment shown in FIG. 1 is the GEHealthcare Engstrom ventilator, although other types of ventilators arecontemplated as being within the scope of the present disclosure.

During operation of the ventilator 12 when the nebulizer 24 is inactive,the ventilator 12 creates a gas flow rate from the ventilator F_(vent)which is delivered to the patient through the inspiratory limb 14. Whenthe nebulizer 24 is inactive, the inspired gas flow F_(insp) deliveredto the patient's lungs 52 through the patient conduit is equal to theventilation gas flow rate F_(vent) and is directly controlled by theventilator 12. During exhalation by the patient 10, the gas flow F_(exp)through the expiratory limb 18 passes through the expiratory flow sensor46. During the operation of the ventilator 12, the sum of the flowsmeasured by the inspiratory flow sensor 44 and the expiratory flowsensor 46 provides an indication at any given point in time themagnitude of the gas flow and in which direction the gas is flowing.Inspiratory flow (toward the patient) is considered positive andexpiratory flow (away from the patient) is considered to be negative.Net flow from the patient can be represented as follows:

F _(net) =F _(insp) +F _(exp)  Equation 1

If there is a constant flow of gas through the circuit and none of thegas is moving toward or away from the patient, the net flow valueF_(net) will be zero.

The control unit 40 can determine the volume of gas delivered toward thepatient from the ventilator by accumulating all net flow F_(net) that isin the positive direction over a breath period. The control unit 40 candetermine the volume of gas returning from the patient V_(exp) byaccumulating all net flow F_(net) that is in the negative direction overthe same breath period. The control unit 40 can determine the volume ofgas delivered toward the patient from the ventilator V_(vent) byaccumulating all net flow F_(net) that is in the positive direction overthe same breath period. The net volume V_(net) is calculated utilizingthe following equation:

V _(net) =V _(vent) +V _(exp)  Equation 2

Without any external gas flow, the volume of gas in the expiratory limb,V_(exp) should equal the volume of gas from the ventilator V_(vent) sothe net volume will be approximately zero.

As can be understood by the above description, the calculation of flowrate and volume are related to each other. The relationship between theflow rate and volume is important in understanding the descriptionbelow.

In the embodiment shown in FIG. 1, when the nebulizer 24 is activated todeliver a nebulized agent to the patient, the nebulized agent isentrained in a flow of gas leaving the nebulizer 24 through thenebulizer conduit 26, as indicated by F_(neb). Since the gas flow fromthe nebulizer enters the inspiratory limb 14 downstream from theinspiratory flow sensor 44, the nebulizer gas flow is unmeasured and thecontrol unit 40 is unaware of the actual flow rate of gas reaching thepatient. In the embodiment shown in FIG. 1, the actual flow of gasreaching the patient F_(insp) is a combination of the gas flow from theventilator F_(vent) and the gas flow from the nebulizer F_(neb), as canbe represented by the following equation:

F_(insp) =F _(vent) +F _(neb)  Equation 3

As can be understood by the above equation, if a user sets a desired gasflow to reach the patient in the ventilator 12, although the gas flowfrom the ventilator can be controlled by the control unit 40, the actualflow rate reaching the patient F_(insp) will be elevated due to the flowof gas F_(neb) from the nebulizer. It is desirable to thus operate theventilator 12 to compensate for the external flow of gas from thenebulizer F_(neb) such that the flow of gas to the patient F_(insp) ismaintained at the desired level.

The net flow of gas F_(net) during a breath cycle can be written by thefollowing equation, which takes into account the nebulizer gas flowF_(neb):

F _(net)=(F _(vent) +F _(neb))+F_(exp)  Equation 4

As can be understood by the above equation, the flow rate of gas fromthe nebulizer F_(neb) can be determined by the control unit based uponthe measurements taken by the inspiratory flow sensor 44 and theexpiratory flow sensor 46 taken over the same sample time period.

Once the flow from the nebulizer F_(neb) has been determined, controlunit 40 can then utilize Equation 3 to reduce the flow from theventilator F_(vent) such that the inspired gas flow F_(insp) reaches thedesired value based on an input from the user in the ventilator 12.Typically, a user will specify a desired tidal volume for the patient byentering the desired tidal volume through the user input 50 which is incommunication with the control unit 40. The tidal volume is a volume ofgas to be delivered to the patient during each breath cycle. Based uponthis desired tidal volume and other breath settings entered by the user,the control unit 40 can calculate a flow rate F_(insp) required toobtain that volume over the inspiratory period of the breath cycle.Based upon the desired inspiratory gas flow F_(insp), the control unit40 compensates for the external gas flow from the nebulizer F_(neb) andreduces the flow of gas from the ventilator F_(vent) by an amountcorresponding to the determined nebulizer gas flow.

Gas flow rates are represented as volume per unit time (e.g. mL/min andL/sec) so the net volume over some sample periods can be used todetermine the external flow rate of the nebulizer. Net volume isexpected to be at its minimum at the end of each complete breath periodand therefore external flow estimates are typically updated at the endof each breath period. In the embodiment shown, the ventilator 12 willaverage the estimated external flow rates over a series of consecutivebreaths. Typically, enough breaths are included in the average such thatthe sample period is at least 20 seconds to smooth out anybreath-to-breath fluctuation in the external flow estimates.

By utilizing the method and system described above, the output of theventilator F_(vent) can be reduced to meet the inspiratory volumerequirements F_(insp) by including the flow of gas from the nebulizerF_(neb). The volume of gas being delivered to the patient will be a moreaccurate representation of the desired volume to be delivered and alarmsincluded in the ventilator 12 will be less sensitive and the displayedvalues on the display 48 will become more accurate.

The use of the inspiratory flow sensor 44 and the expiratory flow sensor46 contained within the ventilator 12 is more desirable than positioninga flow sensor in the inspiratory limb 14 downstream from the nebulizer24. The gas flow leaving the nebulizer includes a nebulized medicalagent, which can coat hotwire flow sensors and lead to inaccurate andunreliable measurements. Since the flow sensors 44 and 46 are containedwithin the ventilator, no additional components are needed to operatethe ventilator in the compensation mode previously described.

FIG. 2 illustrates one method of operating the control unit tocompensate for the external gas flow from the nebulizer. As shown, thecontrol unit 40 first determines in step 54 whether the control unit 40should operate within the compensation mode. The user can select thecompensation mode through an input at the user input 50.

If the control unit 40 determines in step 54 that the ventilator shouldbe operated in the compensation mode, the control unit first determinesthe desired flow rate for the patient (F_(insp)) in step 56. As setforth previously, the user typically enters a desired tidal volume foreach breath through the user input device 50. Typically, the desiredtidal volume for the patient is selected based upon the size of thepatient, the condition of the patient and other parameters that aretypically used to determine ventilation rates, as is well known to thoseof ordinary skill in the art. Based on the desired tidal volume, thecontrol unit 40 calculates the desired flow rate needed to deliver thetidal volume.

Once the desired flow rate for the patient has been determined, thecontrol unit 40 operates the ventilator 12 to deliver the ventilationflow rate (F_(vent)) where the ventilation flow rate is initially equalto the desired flow rate (F_(insp)). When operating the ventilatoraccording to one contemplated embodiment, the control unit 40 initiallyassumes that no additional flow is being added to the flow from theventilator (F_(vent)) such that the flow from the ventilator (F_(vent))is assumed to be equal to the inspiratory flow (F_(insp)) to thepatient.

Although the above method of operating the ventilator may be used, if anexternal flow of gas is present, such as the nebulizer flow rateF_(neb), the patient would receive the nebulizer gas flow on top of theventilator gas flow for at least the first breath, which could lead toover-ventilation of the patient until the nebulizer gas flow iscalculated. In an alternate embodiment, the control unit assigns areference value for the nebulizer gas flow at the start of operation andthe ventilator gas flow (F_(vent)) is reduced by the reference gas flowrate assigned to the nebulizer. In this manner, the initial gas flowrate supplied to the patient may be less than the desired flow rate(F_(insp)) when no external gas flow rate is present. However,under-ventilation of the patient has been found to be more desirablethan an over-ventilation. It is contemplated that the operator of theventilator can enter the reference flow rate into the control unit basedon the expected flow rate from the external gas source, such as thenebulizer. The control unit will use the expected flow rate from theventilator as the reference flow rate until an actual estimation of thenebulizer flow rate is made.

Once the ventilator begins to deliver the ventilation gas to thepatient, the control unit 40 then monitors the expiratory flow rate(F_(exp)) from the patient through the expiratory flow sensor 46, asindicated in step 60. As previously described, if the nebulizer 24 isnot operating, the net volume (V_(net)), as indicated by Equation 2above, will be approximately zero.

However, if the nebulizer 24 is operating to deliver a nebulized medicalagent, the calculated net volume (V_(net)) of gas will not be zero. Asdescribed, the net volume represents the volume of gas from thenebulizer (V_(neb)). The net volume of gas is determined over a sampleperiod, such as a single breath or a series of breaths, and isnormalized to a minute such that the control unit 40 can calculate theflow rates that are used throughout the control of the ventilator.Typically, the flow rates are determined as mL/min. In step 62, thecontrol unit sets the nebulizer flow rate (F_(neb)) to be equal to thenet flow rate (F_(net)).

Once the control unit 40 determines the nebulizer flow rate (F_(neb)),the control unit reduces the target flow rate (F_(insp)) by the flowrate of the gas from the nebulizer (F_(neb)), which is used as theupdated ventilator flow rate (F_(vent)). In this manner, the desiredflow rate to the patient (F_(insp)) returns to its desired value, asindicated by step 64. As described, the control unit compensates for theexternal flow from the nebulizer 24 to adjust the flow rate from theventilator such that the desired inspiratory flow rate reaching thepatient remains at the desired value. Without the use of the externalflow compensation, the inspiratory flow rate to the patient is elevatedby the flow rate of gas from the nebulizer, which results in the patientreceiving a greater than desired gas flow.

In one contemplated embodiment, the control unit limits the value of theestimated nebulizer flow rate (F_(neb)) based on the estimate enteredinto the ventilator. The limit on the value of the nebulizer flow rateprevents the control unit from reducing the ventilator gas flow(F_(vent)) too far, which could result in inadequate flow to the patientfor ventilation. Since the estimated nebulizer flow rate is based on theinspiratory and expiratory sensors, which may suffer from sensor drift,limiting the estimated value for the nebulizer flow rate insures that atleast a minimum gas flow rate is delivered by the ventilator. Theminimum flow rate depends on the desired flow rate, which is based onthe desire tidal volume entered by the user.

Although step 64 contemplates a reduction in the flow of gas from theventilator (F_(vent)) by the flow of the gas from the nebulizer(F_(neb)), it should be understood that the control unit could need toincrease the flow of gas from the ventilator (F_(vent)) during theoperation of the ventilator, such as when the nebulizer is turned off orwhen the flow of gas from the nebulizer decreases. Thus, the controlunit adjusts the flow of gas from the ventilator (F_(vent)) such thatthe combination of the ventilator gas flow and the nebulizer gas flowreaches the desired flow rate to the patient (F_(insp)).

Once the control unit adjusts the ventilator flow rate in step 64, thecontrol unit returns to step 58 and operates the ventilator to deliverthe adjusted ventilation flow rate (F_(vent)). The control unitcontinues to operate steps 58-64 to deliver the desired flow rate of gasto the patient in the manner described.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to make and use the invention. The patentable scope of the inventionis defined by the claims, and may include other examples that occur tothose skilled in the art. Such other examples are intended to be withinthe scope of the claims if they have structural elements that do notdiffer from the literal language of the claims, or if they includeequivalent structural elements with insubstantial differences from theliteral languages of the claims.

1. A method of operating a ventilator to provide an inspiratory gas flowto a patient including an external gas flow from a medical deviceseparate from the ventilator, the method comprising the steps of:setting a desired inspiratory flow rate in a control unit of theventilator; monitoring a flow of gas from the ventilator in the controlunit of the ventilator; monitor an expiratory flow of gas from thepatient in the control unit; determining an external flow rate in thecontrol unit based on the difference between the expiratory flow rateand the ventilator flow rate; and operating the control unit to adjustthe ventilator flow rate based upon the determined external flow ratesuch that the combination of the adjusted ventilator flow rate and theexternal flow rate provides a desired inspiratory flow rate to thepatient.
 2. (canceled)
 3. The method of claim 1 wherein the flow of gasfrom the ventilator and the expiratory flow of gas are determined bysensors associated with the ventilator.
 4. The method of claim 3 whereinthe external gas is injected into the flow of gas from the ventilatordownstream from the sensor contained within the ventilator.
 5. Themethod of claim 1 wherein the external gas is provided by a nebulizerthat provides a nebulized agent in a nebulizer gas flow.
 6. The methodof claim 1 wherein the flow of gas from the ventilator and the 6.expiratory flow rate are determined by sensors external to theventilator and coupled to the control unit of the ventilator.
 7. Themethod of claim 1 wherein the desired inspiratory flow rate for thepatient is determined in the control unit of the ventilator based uponuser input.
 8. The method of claim 7 wherein the desired inspiratoryflow rate is determined through the user input entered into a user inputdevice of the ventilator, wherein the user input device is coupled tothe control unit.
 9. The method of claim 5 wherein the nebulizeroperates independently from the operation of the ventilator and thecontrol unit.
 10. A method of adjusting a flow rate from a ventilator tocompensate for a gas flow from a nebulizer to provide a desiredinspiratory flow rate to a patient, comprising the steps of: setting adesired inspiratory flow rate in a control unit of the ventilator;monitoring the flow of gas from the ventilator in the control unit ofthe ventilator; monitoring an expiratory flow of gas from the patient;determining a nebulizer flow rate in the control unit based upon thedifference between the expiratory flow rate and the ventilator flowrate; and operating the control unit to adjust the ventilator flow ratesuch that the combination of the nebulizer flow rate and the ventilatorflow rate reaches the desired inspiratory flow rate.
 11. The method ofclaim 10 wherein the flow of gas from the ventilator and the expiratoryflow are determined by sensors contained within the ventilator.
 12. Themethod of claim 11 wherein the nebulized agent is injected into the flowof gas from the ventilator downstream from the sensor contained withinthe ventilator.
 13. The method of claim 10 wherein the desiredinspiratory flow rate for the patient is determined in the control unitof the ventilator based upon user input.
 14. The method of claim 13wherein the desired inspiratory flow rate is determined based on adesired breath volume entered into a user input device of theventilator, wherein the user input device is coupled to the controlunit.
 15. The method of claim 10 wherein the nebulizer operatesindependently from he operation of the ventilator and the control unit.16. The method of claim 10 further comprising the steps of: assigning, areference flow rate to the nebulizer flow rate prior to determination ofthe nebulizer flow rate; reducing the flow rate from the ventilator bythe reference flow rate; and providing the reduced ventilator flow rateto the patient prior to determining the nebulizer flow rate. 17-20.(canceled)